The Locktons are Loyalists, meaning that they supporters of the British out of self-interest because they are doing very well in current political circumstances.
Mr. Lockton is a wealthy merchant and benefits greatly from continued British rule of the American colonies. As a result, he's extremely hostile to the Patriots, whom he perceives as a threat to his business interests.
At dinner one evening, he complains how much the Boston uprising has cost him. As a man who does business in New York, Charleston, and Boston, Lockton becomes worried that the contagion of rebellion will spread throughout the colonies, potentially causing further damage to his business.
Like a lot of the social elite in colonial America, the Locktons have thrown in their lot with the British Crown in the hope that they will continue to enjoy security and prosperity. However, in due course, they soon discover that they've bet on the wrong horse.